drm/i915: hw state readout for i9xx dplls

In addition to existing stuff we also need to track DPLL_MD on gen4
and vlv. This is prep work so that we can move the dpll enable
sequence out from the ->mode_set callback into the crtc enabling
functions.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>